Cathode-ray tube and method of making same



June 1, 1954 A. L. BURTON CATHODE-RAY TUBE AND METHOD OF MAKING SAMEFiled NOV. 17, 1950 ALLAN L.

INVENTOR BURTON ATTORNEY Patented June 1, 1954 UNITED STATES PATENTOFFICE CATHODE-RAY TUBE AND METHOD OF MAKING SAME Allan L. Burton,Thompson,

American Optical Mass., a. voluntary ass Application November 17, 1950,Serial N 0.

4 Claims.

method of improving the observation qualities of such images.

or effectively introducing no change as to contrast discrimination ofthe image produced by the inner phosphor layer of such a tube.

Another object is to provide a cathode ray tube or the like with Anotherobject is to provide a tube of the above character with a lightdiffusing surface substantially in the plane of the image-producingsurface whereby the definition of the image visible through thediifusing surface will be effectively unaltered.

Cnn., assignor to Company, Southbridge, ociation of Massachusetts sionhas been the presence of annoying surface reflections as well asspecular reflections of outside sources of illumination from the outerand inner surfaces of the tube.

Many attempts have been made to obviate such reflections as, forexample, by

the combining of an outer reflection-reduction coating with an innerlight-diffusing coating to overcome the difiiculties of the prior art asset forth above and which have been found to provide very sim ple,eflicient and economical means for accomplishing the desired resultswhile introducing effectively no change as to the definition,discrimination and contrast of image.

Referring to the drawings wherein like characters of reference designatelike parts through" out the several views, in Figs. 1 and 2 there isillustrated a cathode ray tube embodying a transparent face portion 19,conical side walls ii and a neck portion 12. The side walls ll may beformed either of glass or metal as desired. The neck i2 contains aconventional electrical discharge device (not shown) which is adapted todirect a moving beam of electrons E toward the face portion of the tubeit so that the electrons will engage a fluorescent screen 13 depositedby conventional methods upon the inner surface i l of the face of thetube. The fluorescent screen 13 is formed of a plurality of tinyphosphor particles each of which is capable of fluorescing whencontacted by the electron beam E.

in following the teachings of the present invention, the inner surfaceill of the face of the tube iii is initially provided with a lightdiffusing surface whether by grinding, sand blasting, acid etching, orby any other known method prior to the depositing of the phosphorcoating l3 thereon. The light dilfusing surface it, therefore, is insubstantially the same plane as the image produced by the phosphorcoating l3 when bombarded by the electron stream E and therefore avoidsdeterioration of the definition of the resultant image as would be thecase if the light diffusing surface were placed on the outer surface ofthe face it and spaced considerably from the plane of the image by theinherent thickness of the material of the face. This light diffusingsurface eliminates spherical reflections which might result from lightfrom an outside source.

To further eliminate specular reflection as well as other surfacereflections from outside sources, the outer surface of the face ill isprovided with a reflection-reduction coating l5. This coating may beformed as follows:

By evaporation of calcium or other fluorides on the outer surface of theglass face ill or by wetting the face with silicic acid and treatingwith hydrochloric acid to precipitate the silicic acid; another type ismade as a film of barium stearate which is produced by dipping the facein water containing barium salts and having a layer of stearic acid ontop; others are films of magnesium fluoride, calcium fluoride, sodiumfluoride or sodium aluminum fluorides; still others are chemicaldecompositions of salts.

The preferred reflection-reduction coatings,

however, are formed in accordance with the teachings of Moulton PatentNo. 2,432,484, issued December 9, ran, and Moulton application SerialNo. 739,545, flled April 5, 1Q47, and comprising compositions consistingof colloidal suspensions containing from about 0.1 to 6.0 per cent byweight of submicroscopic, microgranular, discrete particles of solidanhydrous transparent material such as silica, magnesium fluoride,lithium fluoride, strontium fluoride, calcium fluoride or cryolitesubstantially uniformly dispersed in a volatile liquid inert to theparticles, with the particles being approximately spherical in shape andsubstantially less than one-quarter of the wave length of light indiameter.

The glass surface can be provided with the reflection-reduction coatingby applying-to the surpriate index face a thin layer of the abovecomposition and causing it to dry, leaving a dry coating of very minuteultra-l icroscopic particles on the surface.

It is desirable to control the concentration of the particles in thesuspension to produce a reflection-reduction coating having a resultantthickness of approximately one-quarter wave length of light.

The reflection-reduction coating thus formed will comprisesub-microscopic, discrete, microgranular, transparent solid particleswhich are so deposited on the surface as to form minute projectingirregularities on said surface, the concentration of the particles inthe irregularities decreasing from the surface of the glass outwardlyand the material of the particles being such that the effective index ofrefraction of the reflectionreduction coating varies from substantiallyunity at the layer-air interface to an index value which progressivelyincreases as it approaches the surface of the glass where itsubstantially approximates the index of refraction of said glass.

in order to render the above coating more resistant to abrasion, a smallamount of tetraethylorthosilicate may be incorporated in the colloidalsub-microscopic suspension.

Other reflection-reduction coatings can be formed by following theteachings of Cartwright et al. Patent Nos. 2,207,656 issued July 9,1940, 2,281,474 issued April 28, 1942, and 2,281,475 issued April 28,1942.

For example, a reflection-reduction coating suitable for the purpose canbe formed in accordance with the above patents by applying to the glasssurface a layer of a suitable substance having an index of refractionintermediate the index of refraction of the optical element and theindex of refraction of air and having an optical thickness approximately122/4 of the wave length of preselected light, a: being a positive oddinteger not greater than 9 and preferably being 1. A film of lithiumfluoride, sodium fluoride, sodium aluminum fluoride, calcium fluoride,or the like, of the appropriate optical thickness and approofrefraction, on the surface of the glass will very greatly diminish.light reflection from the surface. Such a film can be deposited byevaporation onto the. glass, with the evaporation being controlled so asto produce a layer with an optical thickness of approximately,onequarter of a wave length of the light and with an eifective index ofrefraction approaching the square root of that of the glass to which itis applied. The fllm can be improved with respect to ruggedness andtenacity by subsequently baking at a temperature between 390 C. and 500C. for a period of time sufficient to effect the desired improvement.

Still another method for reducing reflections from the surface of theglass isthe skeletonizing process wherein. the surface is treated with asolution of fluosilicic acid having a quantity of silica dissolvedthereinranging from saturation to about 3 millimoles supersaturation perliter, the treatment being for such a length of time as will produce onthe surface a skeletonized fllm of substantially pure silica of desireddepth and index of refraction, whereby the desired reduction inreflection is accomplished, the depth being indicated by the wavelengthof light predominantly reflected by the surface. The treatment is pref;erably accomplished by immersing the glass surface in the solution whilethe solution is main.- tained under agitation.

Another method of forming areflectionrreduce tion coating is byproviding a solution of fluosiliclc acid having a The abovereflection-reduction coatings, while being efficient as to theelimination of specular reflections and th reflections from outsidemally non-fluorescing particles of the phosphor will be illuminated andresult in the introduction of halation. The reflection-reductioncoating,

Robert Bowling, Barnes.

Although light difiiusing surface may and the image formingcharacteristics thereof destroyed.

reflection-reduction coating deteriorate the same, results in a tube abetter overall performance.

It might be said that slight deterioration of the image which externalsources.

I claim:

the observer is obtained.

2. A cathode ray tube of the character describedhaving a light-difiusingsurface on the from externally of the tube, a layer of fluorescentmaterial disposed over said light-diffusing surouter side in the' formof minute projectingirregularities with the material of the particlesbeing such that the effectiveindex of refraction of thereflection-reduction coating formed thereby varies from substantiallyunity at its air interface to an index value which progressivelyincreases as it approaches the outer surface of said face where itsubstantially approximates the index of refraction of said face wherebysimultaneously transmission of said luminous image through the outerside of the face of the tube is increased and the amount of light fromsaid luminous image reflected back is decreased, and with the amountreflected back being diflused by said light so as to substantiallyincrease contrast and definition the image when viewed by an observer,and said coating on the outer side of the tube face simultaneouslyreducing the reflection of light, by said surface, from exteriorly ofthe tube.

3. A cathode ray tube of the character described having a layer offluorescent material disposed. on its inner surface in intimate contacttherewith and adapted when bombarded by an electron stream to emit aluminous image for transmission through the face of the tube, said innersurface in intimate contact with the layer of fluorescent materialembodying a plurality of @ontiguously related irregularities forminglightdiffusing means located substantially in the same plane as saidimage emitted by the fluorescent material for breaking up specularimages normally reflectedby said side-from exteriorly of the tube whilenot substantially affecting said image aslit is transmittedtherethrcugh, and a transparent coating on the outer side of the face ofthe tube, said transparent coating being of acharactor and having anoptical thickness such as toincreasethe transmission of said luminousimage through the outer side of the face of the tube and saidlightaiiifusing meansbreaking up the portion not transmitted andreflected back whereby the definition of the luminous image emitted. bythe. fluorescent material is substantially improved, and said outercoating simultaneously' reducing the reflection of light from ex!-teriorly of the tube.

4. A cathode ray. tube of the character described having an acid etched,surface on the inner side of the. face thereof forming light-difiusing'means for breaking up specular images norially reflected by said sidefrom externally of the tube, a layer of fluorescent material disposedover said light-diffusing surface in intimate con tact therewith andadapted when bombarded by an electron stream to emit a luminous imagefor transmission through the face of the tube, said light-diffusingsurface by reasonof its'close proximity to the fluorescentm rial havingsubstantially no effect upon the de nition of said image as it istransmitted therethrough, and a transparent coating on the outer side ofthe face of the tube, said transparent coating having an effectiveoptical thickness and refractive index such as to increase thetransmission of said luminous image through the outer side of the faceof the tube, and said light diffusing surface breaking up the portionreflected back by said outer surface whereby improved contrast anddefinition of the luminous image emitted by the fluorescent material ishad and said transparent coating further functioning to simultaneouslyreduce the reflection of light from exteriorly of the tube.

References Cited in the file of this patent UNITED STATES PATENTS

1. THE TUBE FACE OF A CATHODE RAY TUBE OF THE TYPE HAVING A FLUORESCENTSCREEN AND AN ELECTRON GUN FOR BOMBARDING SAID FLUORESCENT SCREEN TOPRODUCE A LUMINOUS IMAGE TO BE VIEWED BY AN OBSERVER, SAID TUBECOMPRISING A PORTION OF RELATIVELY RIGID TRANSPARENT MATERIAL SUPPORTINGON THE INNER SIDE SURFACE THEREOF THE FLUORESCENT SCREEN WHEREBY, WHENIN USE LIGHT FROM SAID LUMINUOUS IMAGE MAY BE TRANSMITTED THERETHOUGH TOBE VISIBLE TO THE OBSERVER''S EYES, SAID TUBE FACE HAVING, ON THE OUTERSIDE SURFACE THEREOF, A COATING OF TRANSPARENT MATERIAL HAVING ANEFFECTIVE INDEX OF REFRACTION AND OPTICAL THICKNESS SUCH AS TOSUBSTANTIALLY INCREASE TRANSMISSION OF LIGHT RAYS STRIKING SAID OUTERSURFACE FROM A LIGHT SOURCE EXTERIORLY THEREOF AND WHEREBY THEREFLECTION OF SAID LIGHT RAYS TO THE OBSERVER''S EYES WILL BE MATERIALLYREDUCED, AND SAID TUBE FACE FURTHER HAVING LIGHT-DIFFUSING MEANS ON SAIDINNER SAID SURFACE WHICH IS SUCH AS TO BREAK UP SPECULAR REFLECTION OFLIGHT FROM SAID LIGHT SOURCE EXTERIORLY THEREOF WHICH MIGHT OTHERWISE BEVISIBLE TO THE OBSERVER''S EYES, SAID LIGHT-DIFFUSING MEANS BEINGDISPOSED BETWEEN SAID INNER SIDE OF THE TUBE FACE AND THE FLUORESCENTSCREEN AND IN INTIMATE CONTACT THEREWITH WHEREBY IT IS IN SUCH CLOSEPROXIMITY WITH THE LUMINOUS IMAGE PRODUCED ON BOMBARDMENT OF THEFLUORESCENT SCREEN, WHEN THE TUBE IS IN USE, AS TO HAVE SUBSTANTIALLY NOEFFECT UPON THE DEFINITION OF THE IMAGE, SAID TRANSPARENT COATING ONSAID OUTER SURFACE OF THE TUBE FACE SIMULTANEOUSLY INCREASING THETRANSMISSION OF LIGHT FROM SAID LUMINOUS IMAGE THROUGH SAID OUTERSURFACE WHEREBY THE AMOUNT OF LIGHT FROM SAID IMAGE REFLECTED BACKTOWARD THE INNER SURFACE OF THE TUBE FACE IS REDUCED, AND SAID REDUCEDAMOUTN REFLECTED BACK BEING BROKEN UP BY THER LIGHT-DIFFUSING MEANS ONSAID INNER SURFACE THEREBY HALATION IS SUBSTANTIALLY ELIMINATED ANDINCREASED DEFINITION AND CONTRAST OF THE IMAGE AS VIEWED BY THE OBSERVERIS OBTAINED.